1. Technical Field
This disclosure relates generally to an improved structure of a spark plug which may be used in internal combustion engines for automotive vehicles and is designed to increase the service life thereof.
2. Background Art
Japanese Patent First Publication No. 11-219771 discloses a spark plug equipped with a center electrode and a ground electrode which defines a spark gap therebetween to produce a sequence of sparks to ignite an air-fuel mixture admitted into a combustion chamber of an internal combustion engine.
The service life of such a type of spark plug usually depends upon a rise in required voltage resulting from an increase in size of the spark gap. Specifically, the size of the spark gap is increased gradually by spark discharge events during the operation of the spark plug, which leads to an increase in voltage required to be developed between the center electrode and the ground electrode for producing the spark. When such a require voltage exceeds an upper limit, the spark plug will reach the end of its life span.
There have been proposed various measures to shape the structure of the center electrode or the ground electrode to decrease an initial spark gap in order to minimize the required voltage.
The decrease in initial spark gap, however, results in an increase in possibility that an initial flame of the air-fuel mixture is quenched, which leads to a failure in igniting the mixture.
Use of material that exhibits a high degree of wear resistance in making a tip of the center electrode or a portion of the ground electrode which faces the center electrode may also be proposed in order to minimize the increase in size of the spark gap. This, however, results in an increase in overall production cost of the spark plug. Chips provided on the top of the center electrode and/or the surface of the ground electrode which faces the center electrode may be increased in diameter to retard the increase in size of the spark gap. This, however, also results in an increase in overall production cost of the spark plug and in a deterioration of ignitability of the mixture in the engine. It further causes a drop in strength of an electric field around the chips, thus increasing the required voltage.
Conversely, an increase in strength of an electric field in the spark gap and enhancement in ignitability of the mixture in the combustion chamber of the engine may be achieved by decreasing the diameter of the chips. This, however, accelerates the wear of the chips, that is, the increase in size of the spark gap, which will lead to a decrease in service life of the spark plug.
It is, therefore, difficult to shape the structure of the center electrode or the ground electrode to prolong the life of the spark plug.
The spark plug, as taught in the above publication, has a protruding annular edge which is formed on an inner surface of a hollow cylindrical housing to define an auxiliary air gap between itself and the center electrode. In the auxiliary air gap, a spark is created to remove carbon deposits on a porcelain insulator of the spark plug. The annular edge faces an outer periphery of the porcelain insulator and is located away from the spark gap toward a base end of the spark plug, thus resulting in a difficulty in increasing the strength of the electric field in the spark gap and decreasing the required voltage which are needed to prolong the service life of the spark plug.